Johnson Belinda P, Zhou Xuan, Ihara Hoya, Dlott Dana D
School of Chemical Sciences and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
J Phys Chem A. 2020 Jun 11;124(23):4646-4653. doi: 10.1021/acs.jpca.0c02788. Epub 2020 Jun 2.
The formation of hot spots in dynamically compressed, plastic-bonded explosives is known to be the primary mechanism by which these materials ignite and initiate, but hot spots are small, fleeting, and hard to observe. Using a microscope equipped with laser-launched, miniflyer plates, we have studied hot spots in small grains of cyclotetramethylene-tetranitramine (HMX) embedded in a polyurethane binder, shocked to about 20 GPa. A nanosecond video with 4 μm spatial resolution is used to observe hot spot formation and growth, while nanosecond optical pyrometry measured temperature. Using individual ∼200 μm nominally single crystals of HMX (HMX-SC), we observed hot spots forming preferentially on corners or edges. These hot spots are about 4000 K. When there are multiple hot spots, the flame propagated along crystal edges, and the crystal is mostly combusted after about 300 ns. Using polycrystalline grains (HMX-PC), 6000 K hot spots are created near internal defects or crystal junctions. However, the thermal mass of the material at 6000 K is quite small, so after those hot spots cool down, the HMX combustion is similar to the single crystals. Comparing a HMX-based polymer-bonded explosive (PBX) to the individual polymer-bonded HMX-SC and HMX-PC grains shows that the myriad hot spots in the PBX are hotter than HMX-SC and colder than HMX-PC, but they persist for a longer time in PBX than in the individual grains.
在动态压缩的塑料粘结炸药中，热点的形成是这些材料点火和起爆的主要机制，这一点已为人所知，但热点很小、转瞬即逝且难以观察。我们使用配备了激光发射微型飞片的显微镜，研究了嵌入聚氨酯粘结剂中的环四亚甲基四硝胺（HMX）小颗粒中的热点，这些颗粒受到约20吉帕的冲击。利用具有4微米空间分辨率的纳秒级视频来观察热点的形成和生长，同时用纳秒级光学高温测定法测量温度。使用单个标称尺寸为约200微米的HMX单晶（HMX-SC），我们观察到热点优先在角或边缘形成。这些热点温度约为4000K。当存在多个热点时，火焰沿晶体边缘传播，大约300纳秒后晶体大部分燃烧。使用多晶颗粒（HMX-PC）时，在内部缺陷或晶体交界处附近会产生6000K的热点。然而，处于6000K的材料热质量相当小，所以这些热点冷却后，HMX的燃烧情况与单晶类似。将基于HMX的聚合物粘结炸药（PBX）与单个聚合物粘结的HMX-SC和HMX-PC颗粒进行比较表明，PBX中的大量热点比HMX-SC更热，比HMX-PC更冷，但它们在PBX中持续的时间比在单个颗粒中更长。
